Hybrid vehicle systems may be configured with various transmission components. For example, in power-split transaxle hybrid vehicle systems, the transmission may include a planetary gear system directly coupled to the engine, and further coupled to the wheels via one or more rotating components, such as gears, pinions, and bearings. During vehicle operation, transmission control takes into consideration the speed constraints of the various rotating transmission components to avoid damage to the components.
The inventors herein have recognized a potential issue with such systems. The mechanically-imposed speed constraints of the transmission components may limit the maximum attainable vehicle speed, in particular during engine-off modes of operation. Specifically, when the vehicle is operated in an electric mode with the engine off, and the vehicle is being propelled by energy from a system battery, the maximum allowable vehicle speed is limited to a speed that protects the rotating transmission components from speed-incurred damage. As such, this may degrade the operator's drive experience. To provide higher vehicle speeds, the engine may be fueled and spun. However, this reduces operation of the hybrid vehicle system in the electric mode and degrades fuel economy.
In one example, some of the above issues may be at least partly addressed by a method of operating a hybrid electric vehicle system including a rotating transmission component. In one embodiment, the method comprises, during an electric-only mode of operation, below a threshold vehicle speed, raising vehicle speed without spinning an engine while increasing a rotational speed of the transmission component. The method further comprises, above the threshold vehicle speed, raising vehicle speed without increasing the rotational speed of the transmission component by spinning the engine unfueled. In this way, the maximum vehicle speed attainable when operating in the electric-only mode may be raised.
For example, a hybrid electric vehicle may be configured with a planetary gear transmission. During selected conditions when the vehicle is driven only by an electric motor, a controller may allow the vehicle speed to be increased up to a threshold vehicle speed without spinning the engine. As such, up to the threshold speed, as the vehicle speed increases, the rotational speed of a rotating transmission component (such as a gear component) may also increase. Above the threshold vehicle speed, further rotation of the transmission component may lead to mechanical degradation of the component. Therefore, above the threshold vehicle speed, the controller may allow the vehicle speed to be further increased by spinning the engine unfueled. Based on the configuration of the transmission component with relation to the engine and the wheels, as the engine speed is increased, the rotational speed of the transmission component may be decreased (or maintained at the upper limit), allowing the maximum vehicle speed attainable in the electric-only mode to be raised.
In this way, higher vehicle speeds may be attained while continuing to operate a hybrid vehicle in a fuel-efficient electric mode with power provided from a system battery. By spinning the engine unfueled above a threshold speed, the rotational speed of a rotating transmission component may be maintained at, or lowered below, a speed above which the component may incur mechanical damage. By prolonging the electric mode of vehicle operation without limiting vehicle speed, the operator's drive experience can be improved while providing higher fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.